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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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R = CO2Me; NO2;Δ t > 50°C<br />

(O 2N)2C N<br />

O<br />

OSiMe3<br />

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 655<br />

RHC N<br />

O<br />

OSiMe3<br />

[Me 3SiONO] −N2O3<br />

Me3SiOH<br />

Δt<br />

−Me3SiOH<br />

O2NC N O<br />

(Me3Si)2O<br />

Scheme 3.223<br />

R<br />

RCH2NO2<br />

(Ph3)P<br />

N O<br />

[Me3Si]2O<br />

Ph3P + [C(NO2)NO] −<br />

Three possible mechanistic schemes can be suggested for this process. One<br />

<strong>in</strong>volves elim<strong>in</strong>ation of the proton attached to the β-C atom of nitronate A or<br />

A ′ followed by elim<strong>in</strong>ation of the OSi − group from the <strong>in</strong>termediate anion (cf.<br />

Scheme 3.93). Another mechanism is associated with a 1,4-C,O-transfer of the<br />

proton from the β-C atom of nitronate A to the oxygen atom of the N→O fragment<br />

followed by elim<strong>in</strong>ation of silanol from hemiacetal B. The third mechanism<br />

is based on the concerted elim<strong>in</strong>ation of silanol from the m<strong>in</strong>or cis isomer of<br />

SENA.<br />

On the basis of available experimental data, it is impossible to choose a deÞnite<br />

pathway of elim<strong>in</strong>ation of silanol. However, study of silylation of methyl<br />

β-nitropropionate (411) with BSA <strong>in</strong> the presence of trapp<strong>in</strong>g agents rigorously<br />

proved that silyl nitronate D is <strong>in</strong>itially formed. This compound can be detected <strong>in</strong><br />

the [3 + 2]-cycloaddition reaction with methyl acrylate product (413). If silylation<br />

of AN (411) is performed <strong>in</strong> the presence of ethyl v<strong>in</strong>yl ether, α-nitrosoalkene E<br />

can be successfully trapped <strong>in</strong> as heterodiene a Diels-Alder reaction. Dihydrooxaz<strong>in</strong>e<br />

(414) is formed, <strong>and</strong> its silylation affords isolable product (415).<br />

3.5.3.1.3. 1,4-C,O Elim<strong>in</strong>ation (δ-Elim<strong>in</strong>ation) Silylation of vic<strong>in</strong>al substituted<br />

nitroethanes XCH2CH2NO2 demonstrated that if substituent X is highly electronegative,<br />

the result<strong>in</strong>g SENAs A rapidly elim<strong>in</strong>ate the correspond<strong>in</strong>g trimethylsilane<br />

to give nitroethylene (212) (Scheme 3.225).<br />

If X = AcO, <strong>in</strong>termediate SENA can be trapped by methyl acrylate <strong>in</strong> the<br />

[3 + 2]-cycloaddition reaction (isoxazolid<strong>in</strong>e (416)). If X=Cl, attempts to trap<br />

silyl nitronate failed; however, nitroethylene was detected <strong>in</strong> a Diels-Alder reaction.<br />

By contrast, SENAs, <strong>in</strong> which X=OSiMe3 or NHPh, are quite stable.<br />

Therefore, the substituents X can be arranged <strong>in</strong> the follow<strong>in</strong>g series of <strong>in</strong>creas<strong>in</strong>g<br />

elim<strong>in</strong>ation rates of SiX: Cl > AcO >>PhNH.<br />

The above described elim<strong>in</strong>ation (δ-decay) is an analog of the so-called<br />

β-elim<strong>in</strong>ation of β-functionalized silanes (X–C–C–Si → SiX+C=C).<br />

+<br />

22%<br />

(1)<br />

(2)

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